HTML Tidy
"Has anyone seen a pure .NET HTML to XHTML library (component) like Tidy ?"
[Scott Watermasysk]
I haven't seen one yet, but you could whip one up pretty quickly with the DevHawk HTML reader.
Update: There is also the HtmlDocument stuff by DevComponents.
Update: Don points out (by way of Dare) the sgml reader, which can be used to convert HTML to well formed Xml using the XmlReader API.